The invention relates to a printing press comprising means for modifying hydrophobic and hydrophilic areas of the printing image carrier of the press.
In lithographic or surface printing the carrier for the printing image employed is characterized by the fact that its surface properties differ between printing and non-printing areas. In lithography and more especially in the case of offset printing with dampening systems the water accepting or hydrophilic areas and the water repelling or hydrophobic areas are produced which after dampening and inking with oleophilic printing ink are capable of printing on the material to be printed via a blanket cylinder.
In conventional lithographic printing methods as used so far the printing image carrier is in the form of a thin plate, which is prepared outside the press in a plurality of photolithographic method steps. Prior to printing the plate has to be mounted on the plate cylinder, adjusted and run in. The result is that the press is idle for long periods and waste is produced during set-up of the press for a new run. Any modification of the image to be printed on the paper always entails changing the plate.
Accordingly in the past attempts have been made to develop methods for directly producing an image on the printing image carrier in the printing press. One printing image carrier proposed (see the European Pat. No. 101,266) comprises a hydrophilic and a hydrophobic layer able to be applied in the printing press. A laser beam responsive to encoded printing information is able to remove parts of the hydrophobic layer corresponding to the image to be printing. Every time the image is changed the hydrophobic layer is reformed in the press.
Furthermore systems have been proposed in which the hydrophilisation of the surface of the printing image carrier is produced by electrical charges (see German Pat. No. 3,311,237), by activation of photochromes or of thermochromes (see U.S. Pat. No. 3,422,759) or by a structural modification of semiconducting glasses (see German Pat. No. (2,111,561).
These previous methods require either the precise observation of very tight parameters or a large amount of energy for modifying the printing image carrier. Furthermore, there are doubts as to whether such methods would make it possible to carry out multiple modification of the printing image carrier and whether the printing image carrier would be suitable for long runs.